Попробуйте следующим образом
DatabaseReference mDatabase;
mDatabase = FirebaseDatabase.getInstance().getReference();
DatabaseReference mRef = mDatabase.getReference();
mDatabase.child("users").addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: snapshot.getChildren()) {
String name = postSnapshot.getValue();
//do what you want
}
@Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
}
});